For many years, telecommunications lines such as copper strand telephone cable was extended between locations on above ground telephone poles. While this provided convenient access for line repair and for the addition of new lines to service expanding populations, it nevertheless exposed the lines to progressive deterioration from weather, sunlight, temperature changes, and vandalism. In addition, many consider above ground cables and telephone poles to be unsightly and would prefer that they be buried in underground trenches. However, due in part to the need for periodic maintenance and addition of new lines, and to the size of conventional copper cables, subterranean telecommunications lines proved to be impractical for many years.
Fortunately, the relatively recent development and widespread use of fiber-optic cable technology has provided the capability to transmit vast amounts of information simultaneously along a single fiber-optic strand. One advantage of such technology is that since many independent transmissions can be carried simultaneously on each fiber-optic strand of a multi-strand cable, expansion of telecommunications service can be provided without adding a correspondingly expanding number of new cables. In addition, a fiber-optic cable is reliable and can be orders of magnitude smaller in diameter that a copper cable of comparable capacity. Accordingly, fiber-optic telecommunications cable is uniquely suited for subterranean installation and commonly is so installed.
When installed under the ground, fiber-optic cable must be protected against moisture, kinks, dirt, and other corrosive elements. To provide such protection, the telecommunications industry has developed a variety of systems commonly known as multi-duct fiber-optic conduit. Such systems typically comprise an extended length of conduit that is installed in trenches extending between spaced man holes and covered with dirt. The conduit has an outer casing of PVC plastic pipe or other corrosion resistant material and a plurality of plastic inner ducts disposed in and extending along the length of the outer casing. Each inner duct is sized to house one or more fiber-optic cables. The inner ducts serve to separate cables and to prevent them from bunching or tangling as they are drawn through the conduit system.
Multi-duct conduit of the type described usually is provided in rectilinear twenty foot sections that can be joined together end-to-end in the field to form a continuous length of conduit. Each section is preassembled at a manufacturing facility with its inner ducts in place within their outer protective casing. The sections generally are provided with a female end and a male end. The female end of each section is configured to receive and couple to the male end of a like section for joining the twenty foot sections together in end-to-end relationship.
An inner duct coupling device typically is provided in the female end of each section for coupling, aligning, and sealing abutting ends of the inner ducts of two sections as the sections are joined together. Such coupling devices commonly comprise a coupling body having a plurality of longitudinally extending bores or sockets sized to receive the ends of the inner ducts as two adjacent conduit sections are joined in the field. Sealing means is provided in the bores for sealing the inner ducts at their junctions to prevent seepage of water into the ducts and escape of air from the ducts when plugs and pulling tape are pressure propelled therethrough. Some coupling devices also seal against the interior walls of adjacent joined outer casings in an effort to prevent ground water leakage into the outer casings. Examples of multi-duct conduit assemblies for use with fiber-optic cable are disclosed in U.S. Pat. Nos. 4,834,825 and 5,078,432.
While multi-duct conduit systems have proven somewhat successful in providing protection for subterranean fiber-optic transmission lines, they nevertheless have been plagued with numerous problems and shortcomings inherent in their respective designs. For example, since the protective outer casings of such systems are rigid in order to provide the necessary strength, they cannot be bent around corners to accommodate turns at road intersections and the like. Accordingly, a large variety of prefabricated bends and other specially shaped conduit sections usually must be purchased by an installing contractor and in many instances custom bends must be ordered to accommodate a particular need. Unfortunately, such specially shaped sections typically must be fabricated by hand and can be very expensive in comparison to a straight twenty foot conduit section. Further, the requirement of special bends and the like also necessitates careful and highly time consuming planning in advance to insure that the proper type and numbers of such bends are pre-ordered and available for installation at a job site.
In addition to problems related to bends, the coupling devices for connecting together and sealing adjacent conduit sections in some prior art multi-duct systems have not proven entirely successful in sealing the system against the intrusion of ground water and other corrosive elements. For example, in systems where the outer casing seal is provided between the inner duct coupler body and the interior walls of adjacent outer casings, water can migrate into the small space between the outer casings and the coupler body behind the seals where it can deteriorate the coupler body itself thus breaching the integrity of the seal. In some instances, the seals themselves can be deformed or damaged when the sections are joined thus allowing ground water to seep into the system.
It can thus be seen that there exists a continuing and heretofore unaddressed need for a subterranean multi-duct fiber-optic conduit system that seals reliably and effectively against intrusion of ground water and other corrosive elements into any part of the system and that eliminates the need for expensive and inefficient bends and other specially shaped manually fabricated sections. It is to the provision of such a conduit system that the present invention is primarily directed.